1. Field of the Invention
The invention is directed to a vacuum valve comprising a valve body with a valve orifice, at least one closure member for closing the valve orifice, a first piston-cylinder unit having an opening cylinder space and a closing cylinder space, wherein pressure is applied to the opening cylinder space of the first piston-cylinder unit by a pressure medium over the course of a first part of the opening process when opening the vacuum valve and pressure is applied to the closing cylinder space of the first piston-cylinder unit by a pressure medium over the course of a second part of the closing process when closing the vacuum valve, and a second piston-cylinder unit having an opening cylinder space and a closing cylinder space, wherein pressure is applied to the opening cylinder space of the second piston-cylinder unit by a pressure medium over the course of a second part of the opening process when opening the vacuum valve and pressure is applied to the closing cylinder space of the second piston-cylinder unit by a pressure medium over the course of a first part of the closing process when closing the vacuum valve.
2. Description of Related Art
There are various known constructions of vacuum valves in which opening and closing are carried out in two phases in which different actuators are activated.
For example, L-valves are known in which a valve plate is pressed against a valve seat surrounding the valve orifice in the closed state of the vacuum valve. To open the vacuum valve, the valve plate is displaced from this closed position into an intermediate position in which it is lifted from the valve seat but covers the valve orifice and is further displaced into an open position in which it releases the valve orifice. The displacement from the intermediate position to the open position is carried out in a straight line in a direction at right angles to the axis of the valve orifice. The closing of the valve is carried out in the reverse sequence. When the valve plate runs against the valve seat at an inclination to the axis of the valve orifice, the term “J-valve” is also used.
Aside from L-valves and J-valves in which the movement of the valve plate from its closed position to its intermediate position and from its intermediate position to its open position is carried out by means of separate piston-cylinder units, there are also known L-valves and J-valves diverging from the generic type in which a slot guide is provided in order to tilt a valve rod supporting the valve plate around a swiveling axis and, accordingly, to adjust the valve plate between its intermediate position and its closed position, wherein the valve drive has only one individual piston-cylinder unit.
L-valves with separate piston-cylinder units for moving the valve plate from its closed position into its intermediate position and from its intermediate position into its open position are known, for example, from U.S. Pat. Nos. 6,431,518 B1, 6,416,037 B1, 6,056,266 B, US 2008/0017822 A1, U.S. Pat. No. 6,899,316 B2, and 6,561,483 B2.
The pendulum valve is another type of vacuum valve in which the opening and closing process is also carried out in two steps. When the vacuum valve is opened, the valve plate, after having been moved in axial direction of the valve orifice from its closed position into its intermediate position, is moved along an arc into its open position in which it releases the valve orifice. To this end, the valve plate is arranged at a carrying arm which is fastened to a shaft which is rotatable about its axis and displaceable in its longitudinal direction. In addition to embodiment forms diverging from the generic type in which only one actuator is provided for rotating the shaft and for the axial displacement of the shaft (e.g., U.S. Pat. No. 6,776,394 B2), other embodiment forms are known which provide different actuators for this purpose (e.g., in U.S. Pat. No. 7,413,162 B or US 2007/02 901 57 A1). In this connection, a piston-cylinder unit which is integrated in the valve plate can be provided for axial displacement. A rotary drive is used for rotation around the longitudinal axis of the shaft. A rotary drive of this kind could also be constructed with a piston-cylinder unit as actuator.
Another vacuum valve in which the opening and closing is carried out in two steps is disclosed by U.S. Pat. No. 5,577,707. To close the vacuum valve, a valve plate is displaced in a first step from a position in which the valve orifice is released into a position in which the valve orifice is covered. In the second step, a sliding ring which is mounted so as to be displaceable in a sealed manner relative to the valve body is displaced in axial direction of the valve orifice and tightly pressed against the valve plate. The opening process is carried out in reverse of the closing process. A servomotor is provided as an actuator for displacing the valve plate, and a piston-cylinder unit is provided for displacing the sliding ring.
In order to carry out the movements in the proper sequence in valve valves in which the opening and closing take place in two steps with separate actuators, particularly piston-cylinder units, provided for the two steps, position sensors can be provided which acquire the position of the valve plate or of a part that is moved along with the movement of the valve plate. The outputs of these position sensors are then evaluated by an external control device for driving the control valves actuating the piston-cylinder units in the desired manner.
In the vacuum valve known from U.S. Pat. No. 6,431,518 B1, the correct sequence of movements when opening the vacuum valve is carried out by means of a first control element which is moved along with the first piston-cylinder unit driving the movement of the valve plate from its closed position into its intermediate position. Toward the end of the movement of the piston, this piston opens a pressure line through which pressure medium can subsequently escape from the closing cylinder space of the second piston-cylinder unit. The pressure in the opening cylinder space of the second piston-cylinder unit can subsequently displace the piston of the second piston-cylinder unit in order to move the valve plate from its intermediate position into its open position. When closing the vacuum valve, pressure is applied to the closing cylinder space of the second piston-cylinder unit, and its opening cylinder space is vented, wherein there is a connection line to the opening cylinder space of the first piston-cylinder unit in order to generate a dynamic pressure. This prevents a movement of the piston of the first piston-cylinder unit which is loaded in the closing direction from the start of the closing of the vacuum valve until the movement of the piston of the second piston-cylinder unit is terminated. This vacuum valve makes do without position sensors for the parts which are moved during opening and closing, and a correct sequence of movements is achieved in normal operation. Under special operating conditions, for example, unaccustomed network pressures or when building up pressure after a pressureless state, there may be a risk of unwanted movements owing to the control by means of differential pressure or dynamic pressures.
U.S. Pat. No. 6,056,266 B and US 2007/02 72 888 A1 disclose controlling of the movement sequence of L-valves with separate piston-cylinder units for the two movement steps through differential pressures exclusively. When valves of this kind are out of operation for longer periods of time, malfunctioning may occur due to an increased friction coefficient for the first movement of the piston (slip-stick).